1. Field of the Invention
This invention relates generally to mixer circuit architecture in a wireless portable communication device. More particularly, the invention relates to a mixer having dynamic biasing in which the level of the bias signal supplied to a mixer is varied based on system performance requirements.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication handset. The different modulation and transmission schemes each have advantages and disadvantages.
As these mobile communication systems have been developed and deployed, many different standards, to which these systems must conform, have evolved. For example, in the United States, third generation portable communications systems comply with the IS-136 standard, which requires the use of a particular modulation scheme and access format. In the case of IS-136, the modulation scheme can be 8-quadrature phase shift keying (8QPSK), offset π/4 differential quadrature phase shift keying (π/4-DQPSK) or variations thereof and the access format is TDMA.
In Europe, the global system for mobile communications (GSM) standard requires the use of the gaussian minimum shift keying (GMSK) modulation scheme in a narrow band TDMA access environment, which uses a constant envelope modulation methodology. The GSM communication standard has been further developed into what is referred to as the enhanced data rates for GSM evolution, referred to as EDGE.
One of the advances in portable communication technology is the move toward the implementation of a low intermediate frequency (IF) transmitter and receiver and a direct conversion transmitter and receiver (DCR). A low IF receiver converts a radio frequency (RF) signal to an intermediate frequency that is lower than the IF of a convention receiver. A direct conversion receiver downconverts a radio frequency (RF) received signal directly to baseband (DC) without first converting the RF signal to an intermediate frequency (IF). One of the benefits of a direct conversion receiver is the elimination of costly filter components used in systems that employ an intermediate frequency conversion.
Regardless of the type of transceiver used in the system, one or more mixers are used to upconvert the transmit signal to an RF level and to downconvert the received RF signal. A mixer combines the RF signal with a reference signal, referred to as a “local oscillator,” or “LO” signal. The resultant signal is the received signal at a different, and, in the case of a downconverter, typically lower, frequency. The performance required from the mixer generally increases at increased power levels. For example, at high power levels where linearity and noise requirements are more stringent, the mixer needs to provide increased performance. This requires that high current be supplied to the mixer. However, it is desirable to reduce the current supplied to the mixer at lower power levels, where the linearity and noise requirements of the system are less stringent than at high power levels.
Therefore, it would be desirable to reduce the power consumed by a mixer when linearity and noise requirements are relaxed.